Re: [Xen-devel] [PATCH v3.1 01/15] xen/x86: remove XENFEAT_hvm_pirqs for PVHv2 guests

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Nov 03, 2016 at 01:35:37PM +0100, Roger Pau Monne wrote:
> On Mon, Oct 31, 2016 at 10:32:47AM -0600, Jan Beulich wrote:
> > >>> On 29.10.16 at 10:59, <roger.pau@xxxxxxxxxx> wrote:
> > > PVHv2 guests, unlike HVM guests, won't have the option to route interrupts
> > > from physical or emulated devices over event channels using PIRQs. This
> > > applies to both DomU and Dom0 PVHv2 guests.
> > > 
> > > Introduce a new XEN_X86_EMU_USE_PIRQ to notify Xen whether a HVM guest can
> > > route physical interrupts (even from emulated devices) over event 
> > > channels,
> > > and is thus allowed to use some of the PHYSDEV ops.
> > > 
> > > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > 
> > The patch looks fine now for its purpose, but I'm hesitant to ack it
> > without us having settled on whether we indeed mean to hide all
> > those physdev ops from Dom0. In particular I don't recall this (and
> > the reasoning behind it) having got written down somewhere.
> 
> I'm planning to add the following doc update together with this commit:
> 
> diff --git a/docs/misc/hvmlite.markdown b/docs/misc/hvmlite.markdown
> index 946908e..4fc757f 100644
> --- a/docs/misc/hvmlite.markdown
> +++ b/docs/misc/hvmlite.markdown
> @@ -75,3 +75,38 @@ info structure that's passed at boot time (field 
> rsdp_paddr).
>  
>  Description of paravirtualized devices will come from XenStore, just as it's
>  done for HVM guests.
> +
> +## Interrupts ##
> +
> +### Interrupts from physical devices ###
> +
> +Interrupts from physical devices are delivered using native methods, this is
> +done in order to take advantage of new hardware assisted virtualization
> +functions, like posted interrupts. This implies that PVHv2 guests with 
> physical
> +devices will also have the necessary interrupt controllers in order to manage
> +the delivery of interrupts from those devices, using the same interfaces that
> +are available on native hardware.
> +
> +### Interrupts from paravirtualized devices ###
> +
> +Interrupts from paravirtualized devices are delivered using event channels, 
> see
> +[Event Channel Internals][event_channels] for more detailed information about

Is this a must? This mechanism was designed before vAPIC was present -
and has the inherent disadvantage that:

 1) It can't use vAPIC (it actually has to disable this as it needs to
    turn on VMX interrupt window to do this).

 2) It is hackish. It completly bypasses the APIC and it uses the
    VM_ENTRY_INTR_INFO (suppose to be used for traps).

 3) It is also racy for events that are more than 64 values apart (with
    the old 2 level one). That is you can have this callback vector
    being injected couple of times - as the OS interrupt handler does not
    mask the events.

 4) It causes the guest an VMEXIT (to stop it so that we can tweak
    the VM_ENTRY_INTR_INFO).

If we really want to use it, could we instead use the per-vector
that Paul added? HVMOP_set_evtchn_upcall_vector?

Or perhaps just add events -> MSI-X mechanism and then we can also
do this under normal HVM guests?

Either option would require changes in Linux/FreeBSD to deal with this.
> +event channels. In order to inject interrupts into the guest an IDT vector is
> +used. This is the same mechanism used on PVHVM guests, and allows having
> +per-cpu interrupts that can be also used to deliver timers or IPIs if 
> desired.
> +
> +In order to register the callback IDT vector the `HVMOP_set_param` hypercall
> +is used with the following values:
> +
> +    domid = DOMID_SELF
> +    index = HVM_PARAM_CALLBACK_IRQ
> +    value = (0x2 << 56) | vector_value
> +
> +The OS has to program the IDT for the `vector_value` using the baremetal
> +mechanism.
> +
> +In order to know which event channel has fired, we need to look into the
> +information provided in the `shared_info` structure. The `evtchn_pending`
> +array is used as a bitmap in order to find out which event channel has
> +fired. Event channels can also be masked by setting it's port value in the
> +`shared_info->evtchn_mask` bitmap.

.. Well that is for the 2-level, but the FIFO is a bit different.
> 

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